Type cannot be used as an index type

Question

Go to https://www.typescriptlang.org/play/index.html and paste:

let userTypes = {};
let keys = Object.keys[userTypes];

receive error:

error TS2538: Type '{}' cannot be used as an index type.

Why?

Answer 1

Object.keys returns an array of properties. You can't index an array using an object; you must use a number to represent the offset from the start of the array.

As a equivalent example, what do you expect this code to do?:

var a = [1, 2, 3, 4]
console.log(a[{}]);

It's nonsensical.

Edit: After reading the OP's comments and looking over then code again, I realized that my assessment was wrong. While a problem is that the original code is attempting to index the keys function using an object literal, the real issue is the use of square brackets instead of parentheses. This will work:

let keys = Object.keys(userTypes);

It calls keys with userTypes instead of index with it.